1. Field of the Invention
The present invention relates in general to the measurement of signal-to-noise ratios and in particular to an improved and simplified technique for measuring random noise levels in video waveforms.
2. The Prior Art
Various techniques exist for making video signal-to-noise measurements. Generally these techniques fall into one of two broad categories: one, measurement by video noise meter and, two, estimation of noise strength based on visual observation of the noise waveform on a waveform monitor. Although techniques of the first type have been perfected which afford objective noise measurements of good accuracy, e.g., on the order of .+-.1 dB, such techniques require specialized test devices that are often unavailable under operational conditions at measurement locations. The specialized video noise meters required by these techniques are also comparatively costly, which detracts from their suitability for many applications.
Existing visual observation techniques, while in some instances avoiding the need for special test equipment, are of uncertain accuracy owing to their dependency on the skill and subjective judgment of the observer. One method of this type involves the estimation of the quasi peak-to-peak noise voltage. In practice, it has been found that although a particular individual might consistently obtain results within .+-.1 dB using the quasi peak-to-peak method, errors in the signal-to-noise ratios obtained by a number of observers can in some conditions be quite large. For example, tests reported in the literature of a number of skilled engineers who were allowed to set their own viewing conditions for the quasi peak-to-peak technique have shown a range of variation in the signal-to-noise ratio measurements among the engineers of about 5 to 6 dB.
Improvements over the basic quasi peak-to-peak method may be obtained by the use of a storage oscilloscope under standardized conditions. This technique has the advantage of greatly reducing the effect of the observer's judgment in the accuracy of the reading, but has the correlative disadvantage of requiring a costly, and frequently unavailable, storage oscilloscope.
Another known direct observation method is the tangential noise measurement technique. This measurement is made by displacing the random noise waveform vertically, by the addition of a square wave of variable and known amplitude, to produce two bands of noise separated by a clear, dark interval. The amplitude of the square wave is then progressively reduced until an area of uniform brightness is obtained in the center of the display. The amplitude of the square wave for this critical condition is a measure of the r.m.s. noise voltage, which is directly related to the signal-to-noise ratio. Tests of this technique, as practiced in the prior art, have shown improved accuracy relative to the quasi peak-to-peak technique, with variations in results among a number of observers being on the order of 1.5 dB. The method, however, still involves a subjective judgment of noise trace thickness on the part of the observer, and it therefore may be expected that the signal-to-noise ratio readings produced by certain individuals will differ appreciably from the most probable values.